Antibody production differs in 1o and 2o immune responses. Pretransfusion testing attempts to prevent a 2° immune response by detecting any antibody present, and then ensuring that only antigen-negative red cells are transfused. The antibody screen cannot prevent a primary immune response because, unless autologous red cells or red cells from an identical twin are transfused, the recipient is exposed to foreign antigens.
Because pretransfusion testing in the transfusion service detects only 2o (anamnestic) immune responses, it is useful to review antibody kinetics in the two responses:
What can cause antibody levels to drop in a patient experiencing a delayed hemolytic transfusion reaction?